The present invention relates to an electron beam apparatus and more particularly to a scanning electron microscope suitable for obtaining a high resolution image by detecting backscattered electrons generated from a specimen.
Conventionally, in the scanning electron microscope, a detector (semiconductor or scintillator) different from a secondary electron detector is disposed above a specimen (on the side of an electron source) to detect backscattered electrons. The conventional detector is, however, disadvantageous in that when the accelerating voltage for primary electrons is reduced to a low accelerating voltage of several kilovolts or less, sufficient sensitivity cannot be obtained because energy of backscattered electrons generated from the specimen is comparable to that of primary electrons. This is because a method in which a predetermined voltage is applied to the surface for detection of backscattered electrons to accelerate and detect the backscattered electrons is employed as a method for detection of backscattered electrons at low acceleration. However, in this method secondary electrons are also accelerated concurrently and highly efficient separate detection of backscattered electrons and secondary electrons at low accelerating voltage faces difficulties in principle. Further, because of disposition of the backscattered electron detector on the optical axis, the backscattered electron detector must have a shape which permits the primary electron beam to transmit through the detector and detection of backscattered electrons in a region through which the primary electron beam transmits is impossible in principle.
Further, the conventional scanning electron microscope does not have the function to accurately separate and detect backscattered electrons and secondary electrons generated from the same specimen object and therefore it cannot construct a specimen image due to backscattered electron signal or secondary electron signal and display a specimen image as a result of synthesis of backscattered electron signal or secondary electron signal.
Further, in the conventional scanning electron microscope, the backscattered electron detector must be interposed between the objective lens and the specimen to cause the distance between the objective lens and the specimen to be broadened and hence even when a primary electron beam is desired to be focused for irradiation on the specimen, a condensed beam is broadened by an amount corresponding to the distance between the objective lens and the specimen and the resolution is degraded even if a specimen image is formed by backscattered electrons generated from the specimen.